Wnt signaling provides a promising target pathway for developing novel bone anabolic agents. Most studies to date have supported a model in which autocrine or paracrine Wnt signaling in osteoblast-lineage cells directly controls osteoblast biology. This model however, was challenged by a recent study that concluded that LRP5, a co-receptor for Wnt proteins, does not function directly in osteoblasts, but rather through regulating enteric production of serotonin. This study therefore has cast uncertainty about the physiological relevance of direct Wnt signaling in bone. A major cause for the uncertainty is that genetic deletion of -catenin (an obligatory effector of canonical Wnt signaling) in osteoblasts by 2.3Col1-Cre did not affect osteoblast number or function in postnatal animals. However, previous work in the mouse embryo indicates that Wnt/Lrp5/-catenin signaling may function at a stage before 2.3Col1-Cre becomes active. Directly testing this notion in postnatal life has not been feasible because of the lack of proper genetic tools. We have now developed a novel Tet-on system that allows for gene manipulation in osteoprogenitors specifically in postnatal mice. Therefore, we propose to delete -catenin in osteoprogenitors postnatally to test the hypothesis that -catenin directly regulates bone formation in postnatal life (Aim 1). A second critical barrier to progress in the field is the lack of understanding of the molecular mechanisms that mediate Wnt function in osteoblast-lineage cells. Research has been hindered by the lack of a robust mouse model in which a Wnt protein can be manipulated and assessed for its acute signaling ability in vivo. We have now developed such a model wherein a potent bone anabolic Wnt ligand can be activated in a controlled manner. Therefore, in Aims 2 and 3, we will employ this new mouse model to investigate both biochemically and genetically the signal transduction mechanisms through which Wnt7b induces bone formation in vivo. PUBLIC HEALTH RELEVANCE: Novel strategies are required to safely promote bone formation to treat osteoporosis. Wnt signaling has been known to stimulate bone formation and provides a promising target pathway for developing novel bone anabolic agents, but the underlying molecular mechanisms are not well understood. This proposal is designed to understand the mechanism responsible for the potent bone-stimulating function of Wnt proteins. Research results from this study will provide a molecular framework for developing novel bone-enhancing pharmaceutics.